1. Field of the Invention
The present invention is broadly concerned with improved, preferably prefabricated expansion or crack joint couplers particularly designed for use in joining concrete roadway slabs in order to properly transfer vertical forces between concrete slabs while minimizing stress concentrations experienced using conventional dowels at expansion or crack joints. More particularly, the invention pertains to an expansion or crack joint coupler including a steel which bridges joints between concrete slabs with the ends of the coupler embedded in the slabs. In preferred forms, substantially circular disks are inserted in the coupler and its ends are closed with end caps with a multifunction reinforcing sleeve centrally positioned around the outside of the coupler. Alternatively or additionally, the coupler is filled with a structural material such as concrete.
Our invention keeps the strength and stiffness of the pavement at a joint equal to that in the middle of a pavement slab to the extent possible, while keeping the tensile stress low in the slab at a joint so that in the normal shrinkage and expansion of pavements, adjacent slabs will separate at the joints instead of by developing cracks in the middle of the slabs. In doing this, the new coupler transfers bending moments as well as shearing forces.
One of the main purposes of this invention is to transfer shear forces and bending moments from the loaded slab to unloaded slab as heavily loaded wheels cross the joints or cracks while keeping the differential deflection between the slabs a minimum, and decreasing the vertical deflection of the slabs at the joints.
2. Description of Prior Art
In the construction of concrete roadways, it is common practice to install expansion or crack joint assemblies at spaced locations, so that the completed roadway can properly expand and contract under varying temperature and environmental conditions. Typical expansion or crack joint assemblies make use of a plurality of laterally spaced apart shear transfer devices having elongated force transmission members.
With the presently used expansion or crack joints the PCC (Portland Cement Concrete) slabs usually deteriorate near the crack joints. This deterioration is caused by excessive compressive or bearing stresses between the concrete and the dowels. These cause powdering of the concrete at the surface of the dowels near the joints between the slabs, which in turn creates voids around the dowels into which moisture and de-icing salt water flow causing serious corrosion of exposed steel. The powdered concrete abrades anti-corrosion coatings, which causes even previously coated dowels to corrode. Compressive stresses caused by heavy wheel loads also cause lateral splitting forces along the dowels, another compressive failure problem.
The bending stiffness of currently used steel dowels and other dowels in experimental stages are not great enough to transfer the moments required by current traffic, or to reduce warping at the corners of the slabs.
At extreme temperatures, the steel dowels can be compressed between the concrete slabs, again causing excessive stress in the concrete, and even deforming the dowel and crushing the concrete at the ends of the dowels.
The present invention overcomes the problems outlined above and provides an improved expansion or crack joint coupler. By virtue of a unique outer casing defining an internal chamber and having a structural component inserted in the internal chamber, the improved expansion or crack joint coupler provides increased stiffness, strength, and load transferring ability while reducing weight and stress concentrations.
Broadly speaking, the expansion or crack joint coupler of the present invention has a casing with two ends and a central region therebetween. The casing defines an internal chamber and is designed to be embedded in adjacent concrete slabs and to extend across an expansion or crack joint between the slabs. An internal structural component is inserted into the internal chamber to enhance the properties of the casing.
In a preferred embodiment, two tapered spring disks are inserted into the internal chamber of the casing, which is preferably structural steel pipe. The spring disks are positioned approximately equal distances on opposite sides of the casing center, and each spring disk includes a rigid smaller diameter portion and a compressible larger diameter portion which is ring-shaped. The compressible portion of the disk defines a plurality of slits equal spaced around the circumference of the disk, and the compressible portion preferably face outward toward the end of the casing.
In another preferred embodiment, a single solid disk is centrally positioned in the internal chamber. The disk is rigid and preferably fits tightly in the casing chamber.
In still another preferred embodiment, the casing is filled with a structural material. The structural material is preferably concrete, but can be any material with appropriate stiffness.
There is further provided in the practice of the invention a novel outer reinforcing sleeve and a novel pair of end caps. The sleeve is preferably shorter than the casing and is centrally positioned on the casing. The ends of the sleeve are advantageously rounded to reduce stress concentrations, and if a steel sleeve is used, rubber end rings with rounded corners preferably abut the ends of the sleeve. The end caps close the ends of the casing and preferably define an opening receiving the ends of the casing therein. The end caps can also include a plurality of gripping bumps on their outer surface. Both the sleeve and the end caps are preferably formed with a material that is softer than the casing thereby reducing stress concentrations.
There is still further provided in the practice of the invention a novel expansion or crack joint assembly in which two adjacent concrete slabs are positioned to define a narrow gap therebetween. A sheet is positioned beneath the slabs and covers the gap to prevent pumping effects. Preferably, the coupler described above is embedded in the concrete slabs and extends across the expansion or crack joint.
The invention can be used to connect slabs having expansion joints provided for beforehand; or for those which are cast continuously and then caused to crack at predetermined places (where the couplers are placed before casting) by the use of saw cuts made in the pavement at the proper time after casting the continuous pavement. The invention can be used for expansion joints of concrete structures other than pavements.